Different Levels of Transcriptional Regulation Due to Trophic Constraints in the Reduced Genome of Buchnera aphidicola APS

Abstract

Symbiotic associations involving intracellular microorganisms and animals are widespread, especially for species feeding on poor or unbalanced diets. Buchnera aphidicola, the obligate intracellular bacterium associated with most aphid species, provides its hosts with essential amino acids (EAAs), nutrients in short supply in the plant phloem sap. The Buchnera genome has undergone severe reductions during intracellular evolution. Genes for EAA biosynthesis are conserved, but most of the transcriptional regulatory elements are lost. This work addresses two main questions: is transcription in Buchnera (i) regulated and (ii) scaled to aphid EAA demand? Two microarray experiments were designed for profiling the gene expression in Buchnera. The first one was characterized by a specific depletion of tyrosine and phenylalanine in the aphid diet, and the second experiment combined a global diminution of EAAs in the aphid diet with a sucrose concentration increase to manipulate the aphid growth rate. Aphid biological performance and budget analysis (the balance between EAAs provided by the diet and those synthesized by Buchnera) were performed to quantify the nutritional demand from the aphids toward their symbiotic bacteria. Despite the absence of known regulatory elements, a significant transcriptional regulation was observed at different levels of organization in the Buchnera genome: between genes, within putative transcription units, and within specific metabolic pathways. However, unambiguous evidence for transcriptional changes underpinning the scaling of EAA biosynthesis to aphid demand was not obtained. The phenotypic relevance of the transcriptional response from the reduced genome of Buchnera is addressed.

Scaling laws in bacterial genomes: A side-effect of selection of mutational robustness?

“…Similarly, the number of genes ranges from a few hundred (~600 for B. aphidicola) to more than 5500 for P. aeruginosa. Variations in the functional content of the genomes are also visible at the transcription level: some organisms (e.g., B. aphidicola) are hardly able to regulate their transcriptional activity (Reymond et al, 2006) while others display complex regulation networks made up of thousands of tightly interconnected nodes (Stover et al, 2000). When the sequenced bacterial genomes are considered globally, the diversity of genomic structure in prokaryotes is even more striking.…”

Matching the supply of bacterial nutrients to the nutritional demand of the animal host

“…As a result of its very limited genetic repertoire, Buchnera is metabolically fastidious, and its growth is inferred from analyses in silico to be dependent on host supply of 33 metabolites [18]. This nutritional fastidiousness is paralleled by a loss of regulatory capability [7,9,11], such that the host dictates the Buchnera metabolic function in real time by controlling key inputs to the Buchnera metabolic network (this study). Symbiotic bacteria with much-reduced genomes occur in various insects and other animals [2,24], and we predict that, as in Buchnera, their provisioning of nutrients to their hosts is regulated by the concentration of host precursors.…”

“…Any hypothesized mechanism that integrates EAA production by Buchnera with EAA demand of the aphid host must take into account, first, the small genetic repertoire of Buchnera including the dearth of recognizable signal transduction genes and regulatory sequences [7,24] and, second, the evidence that diet-driven variation in EAA production by Buchnera in the pea aphid is not accompanied by large changes in transcript [9,11] or protein levels (this study). The conclusion from the current study that the synthesis of the EAA methionine is determined by precursor supply is fully compatible with these constraints.…”

“…However, Buchnera has very limited capacity to vary gene expression. Its small genome (less than or equal to 0.64 Mb) has a dearth of recognizable regulatory sequences [7], and its gene expression is generally unresponsive to variation in dietary EAAs [8][9][10][11].…”

Integrated Metabonomic−Proteomic Analysis of an Insect−Bacterial Symbiotic System

“…One factor contributing to this difference might be the changes in the physiology of antibiotic-treated aphids linked to their slow growth rates (such differences would not arise for the dietary comparison because the aphids on diet-L and diet-H grew at comparable rates; see Materials and Methods and ref 41). Additionally, a wider range of aphid physiological systems is expected to be affected by elimination of B. aphidicola than by dietary manipulation.…”